The present invention pertains to a system for controlling the operation of a cutting torch or torches of the type operable for programmed cutting of shapes from a sheet metal workpiece and, more particularly, to a height control system for such a torch.
Metal cutting torches, such as oxy-fuel or plasma torches, are operated in response to programmed control to cut a wide variety of shapes from a metal workpiece, typically a plate supported horizontally below one or more cutting torches. The cutting torch is typically mounted on a carriage for two-axis movement over the workpiece and additionally includes a vertical torch transport mechanism operable to move the cutting torch vertically and perpendicular to the planes of two-axis horizontal movement. For optimal cutting operation, the torch must be positioned at a predetermined height above the workpiece.
There are presently several methods and associated apparatus used to set the initial height of a cutting torch above the workpiece. This is commonly known as initial height sensing for establishing the torch pierce distance. In one such prior art system, the increased load on the motor used to drive the vertical torch transport mechanism is detected as the torch tip is driven downwardly into contact with the plate or other workpiece. A control system responsive to the increase in drive motor current (or some other indicia of a stalled motor such as the absence of encoder velocity pulses) stops the motor and reverses it to raise the torch to the predetermined initial operative height. Another system utilizes a circuit ground signal as the tip of the torch contacts the plate to again stop and reverse the vertical torch transport mechanism. Such a system is relatively simple but requires the use of an electrically conductive torch tip and circuitry designed to tolerate high electrical noise. Non-contact systems are also utilized which typically use a sensor mounted laterally outboard of the torch to detect the surface of the plate or other workpiece at a known fixed distance. Such sensors include capacitive, inductive and optical, but with each of these types of sensor, the point on the plate directly under the torch tip is not detected and, as a result, an error in initial height may result.
During horizontal cutting operation of a torch, the torch tip may engage a vertically displaced cut piece or other obstruction in its cutting path. The torch height control system should also be responsive to such accidental or inadvertent contact to halt horizontal torch movement and, if necessary, raise the torch vertically to clear the obstruction.